Optical fiber interferometer

ABSTRACT

A sensor system is disclosed for measuring small physical perturbations inhe environment using an optical fiber interferometer in the Fabry-Perot configuration operating at maximum sensitivity. A single frequency laser source is focused on one end of a single mode optical fiber with highly polished, highly reflective flat ends. An element responsive to the ambient magnetic or electric field alters the fiber&#39;s optical path length, thereby affecting the intensity of light transmitted through the fiber. A detection and feedback system detects the transmitted light and readjusts the optical path length to one which corresponds to maximum sensitivity.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION

This invention relates generally to optical fiber interferometers andmore particularly to an optical fiber interferometer in the Fabry-Perotconfiguration operating at its maximum sensitivity.

Optical fiber interferometers developed in recent years are generallyeither amplitude sensing or phase sensing. Because of their greatersensitivity, however, phase sensors, especially those which employ aMach-Zehnder arrangement, have typically been preferred. In such anarrangement, shown for example in U.S. Pat. No. 4,524,322 to Lloyd C.Bobb, a laser beam is split, with one part of the beam being transmittedby a reference fiber and the other by a sensing fiber which is exposedto the environment or field of interest. The two beams are subsequentlyrecombined and interfere on the surface of a photodetector. Suitablemeans is provided on the reference fiber for either shifting the opticalfrequency or modulating the phase in order to detect the original phasemodulated signal. While such Mach-Zehnder type optical fiberinterferometers provide greatly enhanced sensitivities, their relativecomplexity of design (i.e. the requirement for a reference fiber, beamsplitters and combiners, etc.) results in a more costly,difficult-to-fabricate interferometer having an increased number ofsources of noise due to the additional components required.

Single fiber interferometers have also been developed. Such aninterferometer in the Fabry-Perot configuration is discussed in U.S.Pat. No. 4,536,088 to Rashleigh et al, and described in an article by S.J. Petuchowski et al (IEEE Journal of Quantum Electronics, Vol. QE-17,No. 11, November 1981, p. 2168) and in another article by Yoshino et al(IEEE Journal of Quantum Electronics, Vol. QE-18, No. 10, October 1982,p. 1624.) These interferometers overcome the complexity of design andfabrication problems of the Mach-Zehnder arrangement, but none of thesereferences disclose using the already-available structure of theinterferometer in a feedback system to operate the interferometer at itsmaximum sensitivity.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an interferometer formeasuring very small physical perturbations in the environment such aschanges in magnetic field and electric field.

Other objects include providing an interferometer which is less costly,easier to fabricate, and more sensitive. Still another object is tomaintain operation of an interferometer at its maximum sensitivity.

Briefly, these objects of the present invention are accomplished by anoptical fiber interferometer in the Fabry-Perot configuration. Lightfrom a single frequency laser source is focused onto one end of a singlemode optical fiber with highly polished, highly reflective flat endsthrough which the light is transmitted. A detector situated at the otherend of the fiber detects the intensity of the transmitted light. Fixedto the fiber is a sensor element which changes in response to anenvironmental physical perturbation to be measured. The change in thesensor element causes a change in either the length or the index ofrefraction of the fiber which changes the interference pattern producedat the other end. A feedback system then readjusts the fiber so that thesystem may operate at its maximum sensitivity.

Other objects, advantages and novel features of the invention willbecome apparent from the following detailed description of the inventionwhen considered in conjunction with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a preferred embodiment of anoptical fiber interferometer for measuring a magnetic field according tothe present invention;

FIG. 2 is a typical graph of transmitted light intensity as a functionof the optical path length as applied to the interferometer of FIG. 1;

FIG. 3 is a schematic illustration of another embodiment of a sensorelement for measuring an electric field according to the presentinvention; and

FIG. 4 is a schematic illustration of another embodiment of theinterferometer for measuring a magnetic field according to theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings wherein like characters designate like orcorresponding parts throughout the several views, there is shown in FIG.1 an interferometer in a Fabry-Perot configuration for measuringmagnetic field F. It includes a single mode optical fiber 10 with highlypolished flat ends 10a and 10b perpendicular to the axis of fiber 10 atits respective ends. A single mode fiber will only propogate light alongits axis. Ends 10a and 10b have a uniform dielectric coating of highreflectivity and low loss. Coherent light from a laser 12 is directedvia a lens 14 into end 10a. A detector 16 abuts end 10b, detects theintensity of light transmitted through fiber 10, and produces anelectrical signal B indicative thereof. The amount of transmitted lightis a function of the length l of fiber 10, its index of refraction n,and its reflectivity at ends 10a and 10b. The transmitted light alsovaries with the optical path length (OPL) of the light in fiber 10according to the following formula: OPL=(2πnl/λ)

where

l=length of 10

n=index of refraction of fiber 10

λ=wavelength of the transmitted light

FIG. 2 shows how the transmitted light intensity varies with OPL. It canbe seen that anything that affects the OPL of the light in the fiberaffects the intensity of transmitted light and is therefore detectable.Thus any change in length, index of refraction, or wavelength willmanifest a change in the light intensity at end 10b.

If a physical perturbation in the environment is allowed to affect onlyone of the parameters, the extent of its effect can be measured as achange in light intensity at end 10b. In the interferometer of FIG. 1the parameter affected is fiber length l. A magnetostrictive sensorelement 18, such as nickel or iron, is contiguously fixed as by bondinga finite length to fiber 10. Element 18 changes length as a function ofmagnetic field strength, and thereby produces a corresponding change inthe OPL of fiber 10. This affects the intensity of transmitted light asshown in FIG. 2. The intensity change is detected and the change inmagnetic field strength thereby determined.

It is desirable to operate an interferometer at its point of maximumsensitivity to the environmental perturbation. For the magnetic fieldmeasurement, this would be at a point where a small change in magneticfield strength results in a large change in transmitted light intensity,for example at point A in FIG. 2. Therefore it is desirable to operatethe interferometer with an OPL that corresponds to point A. Thesensitivity of the device can be increased by making the slope at pointA steeper such as by increasing the flatness and reflectivity of theends 10a and 10b. Since fiber length l is the variable parameter formagnetic field measurement, the length l can be adjusted to keep the OPLat the point A. This is done with a feedback system which uses thesignal B from detector 16 to determine the change in fiber length l, andrestores the length to that which corresponds to maximum sensitivity.Feedback amplifier 20 receives signal B and applies an appropriatefeedback current C to a magnetic coil 24 surrounding element 18. Themagnetic field created by coil 24 restores the original length of sensorelement 18 and thereby the length l of fiber 10. Display 26, connectedto amplifier 20, measures and displays the feedback current C which isindicative of the strength of the magnetic field F.

Referring to FIG. 3, there is shown a piezoelectric sensor element 28,substituted for magnetostrictive sensor element 18, which changes lengthin response to an electric field E. The feedback current C' is connecteddirectly to element 28, and is indicative of the electric field strengthchange.

Alternatively, the present invention includes a separatelength-restoring means. Referring to FIG. 4 the output C" from feedbackamplifier 20 connects to a piezoelectric restoring element 30. The fiberis restored to its original length in the same manner as in the magneticfield interferometer of FIG. 1. Either a magnetostrictive or apiezoelectric restoring element can be used with either amagnetostrictive or a piezoelectric sensor element.

It is apparent that the disclosed invention provides an improved opticalfiber interferometer for measuring very small physical perturbations inthe environment such as changes in magnetic field and electric field. Itprovides increased sensitivity and is less costly and easier tofabricate than existing interferometers. The disclosed invention alsoprovides a method for operating the optical fiber interferometer at apoint of maximum sensitivity.

Other embodiments and modifications of the present invention may readilycome to those of ordinary skill in the art having the benefit of theteachings presented in the foregoing description and drawings.

Therefore, it is to be understood that the present invention is not tobe limited to such teachings presented, and that such furtherembodiments and modifications are intended to be included within thescope of the appended claims.

What is claimed is:
 1. An optical fiber interferometer for measuringenvironmental perturbations comprising:a source of coherent light; alength of optical fiber having two flat ends perpendicular to the axisof said fiber at said ends, said fiber being operatively connected totransmit the light received through one of said ends; detecting meansoperatively connected to receive the transmitted light from the other ofsaid ends for detecting the intensity of the received light and forproducing a signal indicative thereof; sensing means operativelyconnected to said fiber for modulating the optical path length of thelight in said fiber in response to an environmental perturbation;feedback means operatively connected to said detecting means forreceiving the signal therefrom and producing a current at said sensingmeans for adjusting the optical path length in response to the signal;and display means operatively connected to said feedback means formeasuring the current and displaying a value indicative of thecorresponding perturbation.
 2. An optical fiber interferometer accordingto claim 1 wherein said fiber is single mode.
 3. An optical fiberinterferometer according to claim 1 wherein said flat ends are partiallyreflective.
 4. An optical fiber interferometer according to claim 1wherein said sensing means comprises a magnetostrictive elementresponsive to changes in ambient magnetic field strength.
 5. An opticalfiber interferometer according to claim 4 wherein said feedback meanscomprises:electronic means operatively connected to receive and amplifythe signal; and a coil about said element operatively connected toreceive the amplified signal and create a magnetic field adjacent tosaid magnetostrictive element.
 6. An optical fiber interferometeraccording to claim 1 wherein said sensing means comprises apiezoelectric element responsive to changes in ambient electric fieldstrength.
 7. An optical fiber interferometer according to claim 6wherein said feedback means comprises:electronic means operativelyconnected for receiving the signal from said detecting means; andcircuitry operatively connected to said electronic means for passing acurrent through said piezoelectric element.
 8. An optical fiberinterferometer for measuring change in magnetic field, comprising:asource of coherent light; a length of single mode optical fiber havingtwo partially reflective flat ends, said fiber being optically connectedto transmit the light received through one of said ends; detecting meansoptically connected to receive the transmitted light from the other ofsaid fiber ends for detecting the intensity of the received light andproducing a signal indicative thereof; a magnetostrictive elementoperatively connected to said fiber for changing the length of saidfiber in response to a magnetic field, thereby altering the optical pathlength of said fiber and changing the interference condition andtherefore the intensity of the light received by said detecting means;feedback means operatively connected to said detecting means forreceiving the signal therefrom and producing a current to said elementfor adjusting the length of said fiber in response to the signal; anddisplay means operatively connected to said feedback means for receivinga portion of the current therefrom and displaying a value indicative ofthe corresponding magnetic field.
 9. An optical fiber interferometeraccording to claim 8 wherein said feedback means comprises:electronicmeans operatively connected to receive and amplify the signal; and acoil about said element operatively connected to receive the amplifiedsignal and create a magnetic field adjacent to said magnetostrictiveelement.
 10. An optical fiber interferometer for measuring change inelectric field, comprising:a source of coherent light; a length ofsingle mode optical fiber having two partially reflective flat ends,said fiber being optically connected to transmit the light receivedthrough one of said ends; detecting means optically connected to receivethe transmitted light from the other of said fiber ends for detectingthe intensity of the received light and producing a signal indicativethereof; a piezoelectric element operatively connected to said fiber forchanging the length of said fiber in response to an electric field,thereby altering the optical path length of said fiber and changing theinterference condition and therefore the intensity of the light receivedby said detecting means; feedback means operatively connected to saiddetecting means for receiving the signal therefrom and producing acurrent to said element for adjusting the length of said fiber inresponse to the signal; and display means operatively connected to saidfeedback means for receiving a portion of the current therefrom anddisplaying a value indicative of the corresponding electric field. 11.An optical fiber interferometer according to claim 10 wherein saidfeedback means comprises:electronic means operatively connected forreceiving the signal from said detecting means; and circuitryoperatively connected to said electronic means for passing a currentthrough said piezoelectric element.